scheduling algorithems first come first serve scheduling shortest job first scheduling priority...

Scheduling Algorithems

First Come First Serve Scheduling Shortest Job First Scheduling Priority Scheduling Round-Robin Scheduling Multilevel Queue Scheduling Multilevel Feedback-Queue

Scheduling

Round-Robin Scheduling

The Round-Robin is designed especially for time sharing systems.

It is similar FCFS but add preemption concept

A small unit of time, called time quantum, is defined

Round-Robin Scheduling

Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.

Round-Robin Scheduling

Round-Robin Scheduling

If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than

(n-1)q time units.

Round-Robin Scheduling

Performance q large => FIFO q small => q must be large with

respect to context switch, otherwise overhead is too high

Typically, higher average turnaround than SJF, but better response

Round-Robin Scheduling

Multilevel Queue

Ready queue is partitioned into separate queues:

foreground (interactive) background (batch)

Each queue has its own scheduling algorithm

foreground – RR background – FCFS

Multilevel Queue example

Foreground P1 53 (RR interval:20)

P2 17 P3 42

Background P4 30 (FCFS) P5 20

Multilevel Queue

Scheduling must be done between the queues

Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation.

Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR

Multilevel Queue

Multilevel Feedback QueueThree queues: Q0 – RR with time quantum 8 milliseconds Q1 – RR time quantum 16 milliseconds Q2 – FCFS

SchedulingA new job enters queue Q0 which is served FCFS. When itgains CPU, job receives 8 milliseconds. If it does not finish in 8milliseconds, job is moved to queue Q1.

At Q1 job is again served FCFS and receives 16 additionalmilliseconds. If it still does not complete, it is preempted andmoved to queue Q2.

Multilevel Feedback Queue

Multilevel Feedback Queue

P1 40 P2 35 P3 15

5.4 Multiple-Processor Scheduling We concentrate on systems in which the

processors are identical (homogeneous)

Asymmetric multiprocessing (by one master) is simple because only one processor access the system data structures.

Symmetric multiprocessing, each processor is self-scheduling. Each processor may have their own ready queue.

Load balancing

On symmetric multiprocessing systems, it is important to keep the workload balanced among all processors to fully utilized the benefits of having more than one CPU

There are two general approached to load balancing: Push Migration and Pull Migration

Symmetric Multithreading

An alternative strategy for symmetric multithreading is to provide multiple logical processors (rather than physical)

It’s called hyperthreading technology on Intel processors

Symmetric Multithreading The idea behind it is to create multiple

logical processors on the same physical processor (sounds like two threads)

But it is not software provide the feature, but hardware

Each logical processor has its own architecture state, each logical processor is responsible for its own interrupt handling.

Symmetric Multithreading

Operating System Examples Operating

Solaris scheduling Windows XP scheduling Linux scheduling

Solaris Scheduling

Solaris Scheduling Solaries uses priority-based scheduling The default scheduling class for a

process is time sharing The higher the priority, the smaller the

time slice The scheduling policy gives good

response time for interactive process and good throughput for CPU-bound process

It includes 60 priority levels

Solaris Dispatch Table Priority: a higher number indicates a higher

priority

Time Quantum: the time quantum for the associated priority

Time Quantum expired: the priority of a thread that has used its entire time quantum without blocking

Return from sleep: the priority of a thread that is returning from sleeping (such as waiting for I/O)

Solaris Dispatch Table

Windows XP Windows XP schedules threads using a

priority-based, preemptive scheduling algorithm

The values of the priority classes appears in the top row

The left columns contains the values for the relative priorities.

The base priority is the value of the normal relative priority for the specific class

Windows XP

Linux Scheduling The Linux scheduling use a

preemptive, priority based algorithm with two separate priority ranges: a real-time range from 0 to 99 and a nice value ranging from 100 to 140

Lower values indicates higher priority Unlike Solaries and XP, Linux assigns

higher-priority tasks longer time quanta

Linux Scheduling

Algorithm Evaluation

Deterministic Modeling Simulations Implementation

Deterministic Modeling

Deterministic Modeling: Process Burst Time P1 10 P2 29 P3 3 P4 7 P5 12

Deterministic Modeling

Deterministic model is simple and fast. It gives the exact numbers, allowing us to compare the algorithms. However, it requires exact numbers for input, and its answers apply only to these cases.

Simulation

Implementation

Even a simulation is of limited accuracy.

The only completely accurate way to evaluate a scheduling algorithm is to code it up, put it in the operating system and see how it works.